Heer and collaborators created a series of software libraries for interactive visualization: prefuse, flare, Protovis and D3. These frameworks are designed to "represent data to facilitate reasoning", "flexibly construct representations" and "enable representational shifts" or transformations.

Protovis, which I've fooled around with a bit, is a functional domain specific language for data visualization. It's successor, D3 (Data Driven Documents), is an adaptation that increases performance and expressivity by making more direct use of the model (the DOM) inherent in the browser.